Washington: Terming the nuclear deal with Iran as "the worst" agreement ever, US President Donald Trump has assured visiting Israeli Prime Minister Benjamin Netanyahu that Tehran would never be able to build a nuclear weapon.

"The security challenges faced by Israel are enormous, including the threat of Iran's nuclear ambitions, which I've talked a lot about. One of the worst deals I've ever seen is the Iran deal," Trump told reporters at a joint news conference with Netanyahu at the White House on Wednesday.

"My administration has already imposed new sanctions on Iran, and I will do more to prevent Iran from ever developing — I mean ever — a nuclear weapon," Trump said.

He said the American security assistance to Israel was currently at an all-time high to ensure that the Jewish state has the ability to defend itself from many threats.

"Both of our countries will continue and grow. We have a long history of cooperation in the fight against terrorism and the fight against those who do not value human life. America and Israel are two nations that cherish the value of all human life," he said.

"This is one more reason why I reject unfair and one-sided actions against Israel at the United Nations — just treated Israel, in my opinion, very, very unfairly — or other international forums, as well as boycotts that target Israel," he said.

File image of US president Donald Trump. AP

Trump said his administration was committed to working with Israel and common allies in the region towards greater security and stability.

"That includes working toward a peace agreement between Israel and the Palestinians. The United States will encourage a peace and, really, a great peace deal. We'll be working on it very, very diligently," he said, adding that the parties themselves who must directly negotiate such an agreement.

"We'll be beside them; we'll be working with them," he added.

Netanyahu applauded Trump for his strong stand on Iran and radical Islamic terrorism.

"Our alliance is based on a deep bond of common values and common interests. And, increasingly, those values and interests are under attack by one malevolent force: radical Islamic terror," he said.

"Mr President, you've shown great clarity and courage in confronting this challenge head-on. You call for confronting Iran's terrorist regime, preventing Iran from realising this terrible deal into a nuclear arsenal," Netanyahu said.

"You have said that the United States is committed to preventing Iran from getting nuclear weapons. You call for the defeat of ISIS. Under your leadership, I believe we can reverse the rising tide of radical Islam. And in this great task, as in so many others, Israel stands with you and I stand with you," he said.

Netanyahu said in rolling back militant Islam, "We can seize an historic opportunity - because, for the first time in my lifetime, and for the first time in the life of my country", Arab countries in the region do not see Israel as an enemy, but, increasingly, as an ally.

"I believe that under your leadership, this change in our region creates an unprecedented opportunity to strengthen security and advance peace," Netanyahu said.

He said Trump led a very important effort against Iran in the past few weeks and pointed out that there are Iranian violations on ballistic missile tests.

"By the way, these ballistic missiles are inscribed in Hebrew 'Israel must be destroyed'. The Palestinian - rather the Iranian Foreign Minister Zarif said, well, our ballistic missiles are not intended against any country. No. They write on the missile in Hebrew, 'Israel must be destroyed', he said.

Netanyahu said he was seeing a change after Trump took over as the US President and it is clearly evident that he is challenging Iran on its violations of ballistic missiles, imposing sanctions on Hezbollah and making them pay for terrorism that they foment throughout the Middle East and beyond.

"Let me say this very openly: I think it's long overdue, and I think that if we work together - and not just the United States and Israel, but so many others in the region who see eye to eye on the great magnitude and danger of the Iranian threat, then I think we can roll back Iran's aggression and danger," Netanyahu said.

"That's something that is important for Israel, the Arab states, but I think it's vitally important for America. These guys are developing ICBMs. They're developing - they want to get to a nuclear arsenal, not a bomb, a hundred bombs. They want to have the ability to launch them everywhere on Earth, and including, and especially, eventually, the United States," he said.